Cork plugs traditionally used in the art for plugging bottles and containers were punched out of natural cork oak bark. Such cork plugs widely vary in quality as often found in natural products. A choice of cork plugs of acceptable quality is uneconomical because rejected cork plugs account for a substantial proportion. Moreover, the price of natural cork material is increasing in these years. Besides, pitch is always associated with natural cork material and causes some drawbacks of scaling off and leakage.
Under the circumstances, it was proposed to form cork plugs from pressed or bonded cork material obtained by molding cork chips with the aid of a binder. The bonded cork plugs received high reputation with respect to quality consistency, ease of working, sealing tightness, and cost.
Urethane and epoxy base binders were widely used as the binders for bonded cork material. Urethane base binders, however, suffered from the problems of carcinogenicity ascribed to isocyanate and dimensional stability (leakage due to shrinkage). Also, epoxy base binders suffered from the problems of give, resilience, temperature (especially low temperature) stability, and dimensional stability. Thus, both types of binders have many problems concerning important functions for use as plugs.